Impedance measuring instrument



June 9, 1953' G. R. GAMERTSFELDER IMPEDANCE MEASURING INSTRUMENT Filed April 3, 1951 Patented June 9, 1953 IMPEDANCE MEASURING INSTRUMENT George R. Gamertsfelder, Pleasantville, N. Y., assignor to General Precision Laboratory Incorporated, a corporation of New York Application. April 3, 1951, Serial No. 218,980

8 Claims.

This invention pertains to an instrument for measuring the alternating current impedance of ircuit components and more particularly to an instrument adjustable over a wide band of frequencies so that the impedance of any selected component may be determined at any selected frequency or frequencies of applied energy.

Heretofore, instruments employed for this purposehave ordinarily consisted of a four-arm passive component impedance bridge, the unknown impedance being interposed as one of the arms. Such devices, however, necessitate several interdependent adjustments to obtain balance and hence a reading which may be used to determine the impedance of the unknown element. Thus operation is laborious and time consuming and great care must be taken to obtain precise results.

The instant invention, on the other hand, provides an instrument in which only two wholly independent adjustments are required to obtain balance so that results are easily and rapidly obtained. Furthermore these adjustments may be made to read directly in terms of the magnitude and phase angle of the alternating current impedance of the element under test. From these two quantities, here termed [Z] and the resistive component R2 and the reactive component Xz of the impedance at the applied frequency may be determined respectively by the relations One of the purposes of the instant invention is to provide an improved instrumentality for determining the impedance of an unknown element the operation of which requires a minimum of manipulative skill.

An additional purpose of the instant invention is to provide an improved measuring device which may be calibrated to read directly in terms of the magnitude and phase angle of the impedance of the element tested.

A still further purpose of the invention is to provide a device wherein the frequency of the potential utilized for measuring purposes may be varied over a wide range so that the impedance of elements under test may be determined at any desired frequency or at various different applied frequencies.

A further understanding of this invention can 2 be secured from the detailed description and"ac= companying drawings in which: I

Figure 1 is aschematic diagram of the instrument of the invention.

Figure 2 is a view of a condenser used in the circuit of Fig. 1.

Referring now to Fig. 1, two electronic oscillators H and I2 are provided. Oscillator I2 is made variable in frequency from, say, .900 kc. to 1 me. by adjustment of the dual condenser" l3}. while oscillator H has a fixed frequency of, say. 1 me. Although the Colpitts type of oscillator is here illustrated, any of the many well-known types-may be employed instead, it being. preferable to select a type that is stable with regard to temperature changes and changes in supply volttages. If desired the oscillator frequencies may be made as high .as 5 me. without undue manipulation difficulty, and may be as low as several kilocycles. I

The oscillator II is provided with a transformer-coupled output comprising a primary l5 and a pair of secondaries l4 and 43. The secondary it. has its midterminal grounded at 2| and its opposite ends are connected to the input terminals l6 and I! of a phase shifting network.

This phase shifting network is composed of two branches each consisting of a resistor and condenser connected in series but in inverse order so that similar elements are located in the opposite diagonal arms of the bridge networkso formed. As illustrated the resistor 22 and condenser 23 connected in that order between terminals l6 and H forms one branch while the condenser 24 and resistor 26 connected in the order named between the terminals [6 and I! "constitutes the second branch. Preferably the resistors 22 and 26 are made equal as are the capacities of the condensers 23 and 24, and likewise at the frequency of the energy applied from the oscillator I l the reactance of the condensers are equal to the resistances of their associated and condenser in such a network are displaced by angles.

These junction terminals are each connectedto respective separate fixed plates 21; 28, 29, 3-] of a quadrant condenser I8 which also includes a rotating plate I9. This condenser is illustrated in Fig. 2, its form comprising four similar fixed plates in one plane with a single rotor in an adjacent parallel plane. Such a condenser is described in the literature and the exact design of the rotor is completely described in U. S. Patent 2,480,187 issued to the same inventor on August 30, 1949. Therefore the condenser will not be further described. Rotation of the plate I9 of this condenser varies the phase angle of the potential applied to plate I9 through 360 electrical degrees.

While a particular phase shifter has here been described it will be appreciated that other de-- vices such as those employing synchros and the like may be used with equal efficacy, it being only necessary to provide an arrangement so that the potential derived from the oscillator II may be shifted by any amount within the entire 360 range.

The oscillator I2 is in all essential aspects like oscillator H except that it is adjustable in frequency through the medium of adjustable condensers I3. provided with a transformer output consisting of a primary all to which are coupled two secondaries 36 and 44. The output derived from the secondary 36 is impressed on the first grid 39 of a pentode modulator 31. At the same time the output derived from oscillator l I through the medium of the secondary I 2, and after having been shifted in phase by the desired amount in the phase shifting network, is applied to the third grid 38 of the pentode modulator 3? through an amplifier 33. The output of the modulator 31 thus consists of signals having the frequency of the signals generated by the oscillators I I and I2 and signals of the sum and difference of the oscillator signal frequencies. This output after having been filtered to remove all but the dif- Like oscillator II, oscillator I2 is ference frequencies by the low pass filter M is applied to an adjustable voltage divider 42, which acts to adjust the magnitude of the energy impressed on terminal 49.

Similarly an output signal derived from the oscillator 12 by means of the transformer secondary 44 and an un-phase shifted output signal derived from the oscillator II by means of the transformer secondary 53 are applied to the first and third grids respectively of a pentode modulator it. The modulator 46 likewise is provided with a low pass filter network 41 connected in its output circuit so that the signal applied to the terminal 5i through the adjustable voltage divider 48 constitutes a signal, the frequency of which is the difference frequency of the signals generated by oscillators I I and I 2, the magnitude of which is of course determined by the setting of the voltage divider 48.

The signal existing at the terminal 49 will have the same frequency as the signal existing at the terminal 5|, since both are the difference frequency signals derived similarly from the same pair of oscillation generators. Likewise their magnitudes may be adjusted to be equal by adjustment of either or both of the potential dividers 42 and 48.

The frequency of the signal present at these terminals is of course selected by adjustment of the frequency of oscillator I2 by means of the adjustable condensers I3. The relative phase of these signals, however, are made to differ in phase were range from plus to minus 180 depending on the setting of the quadrant condenser of the phase shifter, it being Well known that the phase of a beat frequency signal depends directly on and is variable in accordance with the variation in phase angle of one of the signals which produces such beat frequency signal. Thus the outputs of the voltage dividers 42 and 48 at the terminals 39 and 5i have the same frequency and, when the dividers are set alike, the same voltage magnitude, but have a phase difierence equal to the phase angle added to or subtracted from the output of oscillator I2 by the phase shifter connected thereto.

These outputs constitute two arms of a bridge network the remainder of which is composed of a standard calibrated adjustable resistor 51 having one end connected to the terminal 49 and its other end connected to one terminal of the unknown impedance 5B which in turn has its opposite terminal connected to the terminal 5|.

' Thus the calibrated adjustable resistor 51 forms are grounded at their lower terminals.

The indicator 52 may be of any suitable type sensitive to alternating current in the frequency range to be measured as for example, a crystal rectifier meter, thermocouple meter, telephone receiver, oscilloscope, or the like.

It will be apparent that, when the bridge is balanced and no current flows through the indicator 52, the following equalities are true:

lue. 1 2I l l wherein IEiI and [Ezl are the voltages of the beat frequency generators 3i and 46 across the terminals 4956 and 5I-56, R is the resistance of the resistor 51, |Z| the impedance at the frequency employed of the unknown element 58 under test, 0 is the phase angle of the beat frequency generator 31 referred to generator 66,

and 11 is the phase angle of the element 58 under test. The component terms of Equation 1 are magnitudes of vector quantities and all four electrical bridge components may be considered to be vector or complex quantities, the phase angle of the resistor being zero. Inspection of Equation 1 shows that the bridge can be balanced either by varying R. or by changing the ratio ]E1[ to [E2], in both cases also varying (,5 until it equals 0. g In both operations the null indicator indicates whether the change betters or worsens the balance of the bridge. If the variable resistor is employed to balance the bridge [Ell and IE2] are made equal by adjusting the voltage dividers 42 and 48, and the bridge is direct reading, the

age dividers are, of course, calibrated with suit able scales as is the quadrant condenser. I8."

The resistance and reactance of the impedor 58 at the frequency ofthe test are as stated found from the simple relations Rz=lZ| cos s (3) and XZ=IZI sin (4) in which Hz is the resistance of the impedor and Xz the reactan-ce of the impedor.

What is claimed is:

1. An impedance measuring instrument comprising, first and second sources of alternatin current of the same frequency connected in series across a series network composed of a calibrated adjustable impedance and an impedance element of unknown magnitude, means for adjusting the phase of one of said alternating current sources relative to the other, and a null indicator connected in circuit with said alternating current sources and said impedances for indicating the balance or unbalance of bridge circuit so formed.

2. An impedance measuring instrument comprising, first and second sources of alternating current of the same frequency connected in series, a series network connected in shunt thereto including a calibrated adjustable resistor and an impedance element of unknown magnitude, means for adjusting the phase of one of said alternating current sources relative to the other, and a null indicator connected between the juncture of said resistor and impedance and the juncture of said alternating current sources.

3. An impedance measuring instrument comprising, first and second sources of alternating current, the signal frequencies of which are equal, said alternating current sources being connected in series, a network connected in shunt to said alternating current sources, said network including a calibrated adjustable resistor and an im I pedance the magnitude of which is to be determined, means for adjusting the phase of one of said alternating current sources relative to the other, means for adjusting the relative magnitudes of the energy derived from said alternating current sources and impressed on said network, and a null indicator connected between the juncture of said resistor and impedance and the juncture of said alternating current sources.

4. An impedance measuring instrument comprising, first and second oscillation generators, a first modulator, circuit means for impressing signals generated by each of said first and second oscillation generators on said first modulator whereby a beat frequency signal is obtained, a second modulator, means for impressing a signal generated by one of said oscillation generators on said second. modulator, means for phase shifting a signal generated by the other of said oscillation generators, circuit means for impressing said phase shifted signal on said second modulator, output circuits for said. first and second modulators forming one branch of a four arm bridge circuit, the remaining arms of which consist of an adjustable calibrated impedance and an impedance whose magnitude is unknown, and a null indicator connected between the juncture of said modulator output circuits and the juncture of said 7 adjustable and unknown impedance.

5. An impedance measuring instrument comprising, a first fixed frequency oscillation generator, a second oscillation generator including means for varying the frequency of the signals generated thereby, a first modulator, circuit means for impressing signals generated by each of said first and second oscillation generators on the input of said first modulator, a second modulator, means for impressing a signal generated by one of said oscillation generators on the input of said second modulator, means for varying the phase of a signal generated by the other of said oscillation generators, circuit means for impressing the phase varied signal on said second modulator, said first and second modulators having output circuits connected in series to form one branch of a four arm bridge circuit, an adjustable resistor in the third arm and an impedance whose magnitude is unknown in the fourth arm of said'bridge circuit, and a null'indicator connected. juncture of said adjustable resistor and unknown impedance.

6. An impedance measuring instrument comprising, first and second oscillation generators, a first modulator, means for impressing signals generated by each of said oscillation generators on said first modulator whereby a heat frequency signal is obtained, a second modulator, means for impressing a signal generated by one of said oscillation generators on said second modulator, circuit means including adjustable phase shifting means interconnecting an output circuit of the other of said oscillation generators and said second modulator, output circuits for said first and second modulators including amplitude adjusting means, said output circuits being connected in series to form one branch of a four arm bridge circuit, an adjustable calibrated resistance in the third arm and an impedance whose magnitude is unknown in the fourth arm of said bridge cir cult, and a null indicator connected between the uncture of said modulator outputsand the juncture of said adjustable calibrated resistance and unknown impedance.

An impedance measuring instrument comprising, a first oscillation generator generating osc llatory signals of a fixed frequency, a second oscillation generator including means for varying the frequency of'the signals generated thereby, a first modulator, circuit means for impressmg signals generated by each of said oscillation generators on said first modulator, a second modulator, means for impressing a signal generated by said second oscillation generator on said sec ond modulator, circuit means including adjustable calibrated phase shifting means intercon- V necting an output of said first oscillation generator and said second modulator, output circuits for said first and second modulators including amplitude adjusting means, said output circuits being connected in series and forming one branch of a four arm bridge circuit, an adjustable calibrated resistance forming the third arm and an impedance whose magnitude is undetermined forming the fourth arm of said bridge circuit and a null indicator connected between the junc ture of said modulator outputs and the juncture of said third and fourth arms.

8. An impedance measuring instrument comprising, a first oscillation generator generating oscillatory signals of afixed frequency a second oscillation generator including means for varying the frequency of the signals generated ther by, a first modulator including a pair of input electrodes and an output circuit, circuit means interconnecting the output of one or" said oscillation generators with one of said input elec- 2,6 1,6 2 7 a 8 ulator, an adjustable calibrated phase shifting the juncture. of said calibrated resistance and circuit interconnected between the output of said said impedance. r first oscillation generator and the other input GEORGE R. GAMERTSFELDER. electrode of said second modulator, amplitude adjusting means connected in the output circuit 5 R f n Cited i t file of thi patent of at least one of said modulators, said modulator output circuits being connected in series, an ad-. UNITED STATES PATENTS justable calibrated resistance and an impedance Number Name Date of unknown value connected in series with each 1,847,1 7 Mayer Mar. 1, 1932 other and across said series connected modula- 10 2,146,073 Jennens et a1 -1 Feb. '7, 1939 tor output circuits, and a null indicator connected 2,149,756 Arenberg et al Mar. 7, 1939 between thejuncture of said output circuits and 2,232,792 Levin Feb, 25, 1941 

